There is no firewall active on your linux server. I checked that on the kernel level outside of ispconfig. So you can take this as fact.
You describe that you have lan and wan addresses on the sever. Currently there is only one ip configured on the linux system. So if i understand you correctly, there must be at least two addresses configured on the linux server. In that case, you dont have a problem with the firewall and also not a problem with ispconfig. The problem is then that one ip or network card is missing in your network card configuration. So which and how many network cards are installed in the server and which ip addressescare assigned to these network cards. You might want to send me the details trough the support system, so you dont have to publish them here.